1. Field of the Invention
The present invention relates to a hexavalent chromium-free surface treating agent whose environmental load is reduced, and a Sn-, Sn alloy-, Al- or Al alloy-coated, surface-treated steel sheet treated with the agent, which is used in a fuel tank or an exhaust system of automobiles, or in building materials.
2. Description of the Related Art
Not only the weldability, but also general corrosion resistance on the outside and anti-fuel corrosion resistance on the inside, is required of materials for fuel tanks of automobiles using gasoline as a fuel. Heretofore, Pb—Sn based coated steel sheets obtained by terne coating have commonly been used as materials for fuel tank. However, with regard to environmental problems, regulation of Pb has become severe. The use of alcohol-containing fuels such as a gasoline/alcohol mixed fuel (including M15 containing about 15% by mass of methanol and M85 containing about 85% by mass of methanol) called gasohols have recently been promoted in some countries by exhaust gas regulation after taking environmental problems into consideration. However, since conventional terne coated sheets are liable to be corroded by the alcohol-containing fuels described above, it is of urgent necessity to develop materials for fuel tanks, which are superior in anti-fuel corrosion to alcohol-containing fuels. Due to this tendency, various products including hot-dip aluminized steel sheet, hot-dip Sn—Zn coated steel sheet and the like have been developed as materials for Pb-free automobile fuel tanks.
Japanese Unexamined Patent Publication (Kokai) No. 58-45396 discloses a surface treated steel sheet for fuel tanks obtained by subjecting a steel sheet having a Zn—Ni alloy layer (Ni content: 5 to 50% by mass, a thickness of 0.5 to 20 μm) to a chromate treatment. Japanese Unexamined Patent Publication No. 5-106058 discloses a surface treated steel sheet for fuel tank obtained by subjecting a steel sheet having a Zn—Ni alloy coating (Ni content: 8 to 20% by mass) in a coating mass of 10 to 60 g/m2, to a chromate treatment containing hexavalent chromium. Japanese Unexamined Patent Publication Nos. 10-168581 and 11-217682 disclose materials obtained by subjecting hot-dip aluminized materials to a chromate treatment. As described above, almost all the products which are intended to replace terne coating are products with an outermost layer containing hexavalent chromium obtained by a chromate treatment.
As is well known, hexavalent chromium is a harmful and unfavorable substance because it exhibits carcinogenicity to the human body, has possibility of elution from products and causes problems in waste treatment. Although some products are treated with trivalent chromium as electrolytic chromate, these products use hexachromium during manufacture and are the same as conventional products in that the waste treatment must be conducted. However, there is no appropriate treatment exhibiting a performance which can be used in place of chromate treatment, at present.
As a matter of course, various studies of surface treating agents using no hexavalent chromium have been made. For example, Japanese Examined Patent Publication (Kokoku) No. 2-18982 discloses a surface treated steel sheet, for fuel tanks, comprising a plating layer, as a lower layer, which is made of Zn or contains Zn as a main component, and an upper layer containing a metal powder of a 10% or more Cr-containing stainless steel, Zn, Al, Mg, Ni and Sn in a phenoxy resin and a rubber-modified epoxy resin as main components. However, since the resistance weldability such a spot weldability and seam weldability is required to the use in fuel tank, a treatment which forms a thick organic coating cannot satisfy the requirement.